Carbon nanotube sheet structure and method for its making

ABSTRACT

A carbon nanotube (CNT) sheet containing CNTs, arranged is a randomly oriented, uniformly distributed pattern, and having a basis weight of at least 1 gsm and a relative density of less than 1.5. The CNT sheet is manufactured by applying a CNT suspension in a continuous pool over a filter material to a depth sufficient to prevent puddling of the CNT suspension upon the surface of the filter material, and drawing the dispersing liquid through the filter material to provide a uniform CNT dispersion and form the CNT sheet. The CNT sheet is useful in making CNT composite laminates and structures having utility for electro-thermal heating, electromagnetic wave absorption, lightning strike dissipation, EMI shielding, thermal interface pads, energy storage, and heat dissipation.

BACKGROUND OF THE INVENTION

This invention relates generally to carbon nanotubes, and moreparticularly to methods for forming materials and structures from carbonnanotubes.

The exceptional mechanical properties of carbon nanotubes can be used inthe development of nanotube-based, high performance structural andmultifunctional nanostructural materials and devices. Carbon nanotubeshave been made that are nanometers in diameter and several microns inlength, and up to several millimeters in length. Strong interactionsoccur between nanotubes due to the van der Waals forces, which mayrequire good tube dispersion, good tube contact, and high tube loadingin materials and structures formed from carbon nanotubes.

Many applications, such as electrical conducting, thermal conducting andhigh performance nanocomposites, are made by pre-forming nanotubes intoa network or membrane (5-200 μm in thickness) with controllednanostructures (density, porosity, dispersion, alignment, and loading).These membranes would also make nanotube materials and their propertiescapable of transfer into a macroscale material for easy handling. Thesepreformed nanotube networks are also called buckypapers in theliterature. Buckypapers are produced by a multiple-step process ofdispersing nanotubes into a suspension and filtering the producedsuspension. The produced buckypapers can be easily handled similar toconventional surface veil, carbon fiber, or glass materials. However,all the existing manufacturing techniques for nanotube membranes arediscontinuous or batch processes and can only produce small quantitiesand very short membrane materials, which are serious barriers for futurepractical applications of nanotube membranes.

Current discontinuous or batch techniques can only produce nanotubemembrane materials by filtering a nanotube suspension, and thedimensions are limited by the filter dimension. In these techniques, awell-dispersed nanotube suspension is first prepared, optionally withthe aid of selected surfactant and mixed using high shear mixing and/orsonication. Then, a filtration system with a filter membrane of 0.1 μmto 10 μm pore size is employed to filter the prepared suspension withthe aid of vacuum or pressure. During the filtration, nanotubes depositonto the surface of the filter membrane to form a nanotube network.After filtration, the produced nanotube film or buckypaper can be peeledoff from the filter membrane. Producing large quantities of buckypapersrequires frequent changing of the filters. Current processes use manyfilters to complete the filtration and limit manufacture of thebuckypapers to piece by piece, which is time consuming, costly and alsodifficult to ensure consistent product quality. More importantly, due tothe limitation of filter dimension, the product membranes are of alimited length (usually less than one foot).

U.S. Pat. No. 7,459,121, incorporated herein by reference in itsentirety, describes a method for the continuous production of a networkof nanotubes or other nanoscale fibers. The method comprises making asuspension of nanoscale fibers dispersed in a liquid medium, andfiltering the suspension by moving a filter membrane through thesuspension liquid, such that the nanoscale fibers are deposited directlyon the filter membrane as the fluid medium flows through the filtermembrane, thereby forming a continuous membrane of the nanoscale fibers.In one embodiment, the deposition of the nanoscale fibers occurs whenand where the filter membrane moves into contact with a static, porousfilter element. In another embodiment, the deposition of the nanoscalefibers occurs when and where the filter membrane moves into contact witha dynamic, porous filter element. For example, the filter element can bea rotary element which is mechanically driven to rotate and at leastpartially assist in moving the filter membrane across the filterelement. The carbon nanotubes described therein are preferably singlewall carbon nanotubes, and are described being commercially availablefrom companies such as Carbon Nanotechnologies, Inc. (Houston, Tex.,USA). Though the lengths of such carbon nanotubes are not described inthe patent, the length of carbon nanotubes produced by CarbonNanotechnologies, Inc. is understood to be in a range less than 0.01 mm(less than 50 microns). The apparatus described in U.S. Pat. No.7,459,121 is not believed to be commercially available.

U.S. Pat. No. 7,955,535, incorporated herein by reference in itsentirety, describes forming a suspension of small diameter SWCNTs andlarger diameter MWCNTs (or CNFs) and filtering the suspension to removethe liquid.

U.S. Pat. No. 8,351,220, incorporated herein by reference in itsentirety, describes a nanoscale fiber film comprising a buckypaperhaving an areal density (basis weight) of about 20-50 gsm, and athickness of about 5 to 100 microns.

U.S. Pat. No. 5,254,399 describes a method for forming a nonwoven fabrichaving excellent sheet formation, that comprises fibers having adiameter of 7 μm or less and an aspect ratio (ratio of fiber length tofiber diameter, or L/D) in the range of 2000<L/D≤6000, and optionallythermal bonding fibers, and the fibers being three-dimensionallyentangled, the disclosure of which is incorporated by reference in itsentirety. The fibers can include organic synthetic fibers such aspolyester fiber, polyolefin fiber, polyacrylonitrile fiber, polyvinylalcohol fiber, nylon fiber, polyurethane fiber and the like,semi-synthetic fibers, regenerated fibers, natural fibers and the like.

Notwithstanding, there remains a need for a process that manufacturesCNT sheets on an industrial and commercial scale in order to meet theemerging technological and market needs for such structures.

SUMMARY OF THE INVENTION

Methods and devices are provided herein for the continuous production ofa network of carbon nanotubes (CNTs) into a continuous sheet structures.

The present invention includes a process for forming carbon nanotubes(CNT) structures that includes filtering a volume of a solutioncomprising a dispersion of CNTs, over a filter material to provide afiltered CNT structure having uniform dispersion of the CNTs over thefilter material, and a step of drying the filtered CNT structure into aCNT sheet. CNTs can include single wall CNTs (SWCNTs) or multi-wall CNTs(MWCNTs).

The present invention includes a process for manufacturing a carbonnanotube (CNT) sheet, comprising the steps of: i) applying an aqueoussuspension of carbon nanotubes (CNTs) dispersed in a liquid onto acontinuous, moving, porous material (which acts as a filter); ii)drawing by vacuum or pressure the liquid of the aqueous suspension ofthe CNTs through the porous material, and filtering a uniform dispersionof CNTs over the porous material to form an entangled CNT structure;iii) optionally drying any residual liquid from the entangled filteredCNT structure to form a CNT sheet over the porous carrier material; andiv) removing the CNT sheet from the porous carrier material.

The present invention further includes a continuous process formanufacturing a continuous composite CNT sheet, comprising the steps of:i) applying a continuous porous substrate layer to an upper side ofcontinuous, moving porous carrier material; ii) applying a suspension ofcarbon nanotubes (CNTs) dispersed in a liquid on the porous substratelayer; iii) drawing by vacuum or pressure the liquid of the aqueoussuspension of the CNTs through the porous substrate layer and the porouscarrier material, and filtering a uniform dispersion of CNTs over theporous substrate layer to form an entangled CNT structure; iv)optionally drying any residual liquid from the entangled CNT structureto form a composite CNT-substrate sheet over the porous carriermaterial; and v) removing the composite CNT-substrate sheet from thecontinuous porous carrier material.

The present invention further includes a continuous process formanufacturing continuous CNT sheets, comprising the steps of: i)applying a suspension of carbon nanotubes (CNTs) dispersed in a liquidonto a continuous, moving porous carrier-substrate layer; ii) drawing byvacuum or pressure the liquid of the aqueous suspension of the CNTsthrough the continuous, moving porous carrier-substrate layer, andfiltering a uniform dispersion of CNTs over the continuous porouscarrier-substrate layer to form an entangled CNT structure on thecarrier-substrate layer; and iii) optionally drying any residual liquidfrom the entangled CNT structure to form a continuous compositeCNT-substrate sheet including the entangled CNT structure and thecarrier-substrate layer.

The invention also includes a process for manufacturing a carbonnanotube (CNT) sheet, comprising the steps of: i) applying an aqueoussuspension of carbon nanotubes (CNTs) dispersed in a liquid on acontinuous porous carrier material; ii) drawing by vacuum or pressurethe liquid of the aqueous suspension of the CNTs through the porouscarrier material, and filtering a uniform dispersion of filtered CNTsover the porous carrier material to form an entangled CNT structure,iii) optionally drying any residual liquid from the entangled CNTstructure to form a CNT sheet over the porous carrier material; and iv)optionally removing the CNT sheet from the porous carrier material.

In another aspect of the invention, a method for forming a CNT structureor sheet includes making a dispersion or suspension of carbon nanotubes(CNTs) in a dispersing liquid, the dispersed CNTs forming a medianbundle or agglomeration length of at least 50 microns; passing a volumeof the CNT suspension over a filter material to provide a continuouspool or coating of the CNT suspension over the filter material, having auniform depth (or thickness) sufficient to prevent puddling of the CNTsuspension upon the surface of the filter material; drawing under vacuumor pressure the dispersing liquid through the filter material to providea uniform dispersion of the CNTs over the filter material and forming aCNT structure; drying any residual aqueous liquid from the CNT structureto form a CNT sheet over the filter material; and removing the CNT sheetfrom the filter material. The CNTs can include SWCNTs, including SWCNTshaving a median length of at least 5 microns and an aspect ratio of atleast 2,500:1. The CNTs can also include MWCNTs, including SWCNTs havinga median length of at least 50 microns and an aspect ratio of at least2,500:1

In another aspect of the invention, the step of drawing the dispersingliquid through the filter material comprises passing the CNT-ladenfilter material over a vacuum screen or box for a time sufficient todraw the dispersing liquid through the filter material and vacuum screenor box.

The present invention also includes an apparatus for manufacturing acontinuous CNT sheet, and a continuous process for manufacturingcontinuous CNT sheets employing the apparatus. The continuous processcomprising the steps of: i) moving a continuous conveying belt along apath that traverses a pooling region and a vacuum box; ii) applying acontinuous porous carrier material to an upper side of the continuousconveying belt; iii) applying an aqueous suspension of carbon nanotubes(CNTs) dispersed in a liquid on the porous carrier material, thedispersed CNTs having a median bundle length of at least 0.05 mm and anaspect ratio of at least 2,500:1; iv) forming a continuous pool of theaqueous suspension of the CNTs over and across the width of thecontinuous porous carrier material in the pooling region, the continuouspool of the aqueous suspension of the CNTs having a uniform thicknesssufficient to prevent puddling upon the continuous porous carriermaterial; v) advancing the porous carrier material and the continuouspool of the aqueous suspension of the CNTs over the vacuum box; vi)drawing by vacuum the liquid of the aqueous suspension of the CNTsthrough the porous carrier material, and filtering a uniform dispersionof filtered CNTs over the porous carrier material to form a filtered CNTstructure; vii) optionally drying any residual liquid from the filteredCNT structure to form a CNT sheet over the porous carrier material; andv) removing the CNT sheet from the continuous porous carrier material.The CNTs can include SWCNTs, including SWCNTs having a median length ofat least 5 microns and an aspect ratio of at least 2,500:1.

The CNTs can also include MWCNTs, including SWCNTs having a medianlength of at least 50 microns and an aspect ratio of at least 2,500:1.Said SWCNTs and longer MWCNTs are hereinafter described as “entanglableCNTs”.

In an aspect of the invention, the continuous porous carrier material isa continuous porous film, sheet, or fabric material. The continuousporous carrier material provides a stable and resilient porous structurefor pulling the forming CNT structure through and along the apparatus,to prevent tearing and degradation of the CNT structure duringmanufacture. The continuous porous carrier material can include a wovenor meshed synthetic polymer, which can include hydrophobic polymers,including but not limited to polytetrafluoroethylene (PTFE), also knownas Teflon®, and hydrophilic polymers, including but not limited toaliphatic polyamides, also known as nylon. A metal-coated woven or ametallic mesh or expanded foil or screen material can also be used as aporous carrier material. In an embodiment, the porous carrier material,has a plurality of openings having a size between about 0.1 micron, andup to about 5 mm. A continuous roll of metallic wires or fibers, from aplurality of spools or rovings, can be pulled across the width of thefiltering material in the machine direction. The CNT dispersion can thenbe filtered upon the aligned or unidirectional metallic wires, forming aCNT-metallic wire composite rollstock material. This process is similarto a pultrusion process, but using the CNT dispersion to encapsulate thefibers instead of a resin. Non-limiting examples of a pultrusion processare disclosed in US Patent Publication US 2011/0306718 and U.S. Pat. No.5,084,222, the disclosures of which are incorporated by reference intheir entireties.

In another aspect of the invention, the continuous porous carriermaterial can also be a filter material that filters the dispersed CNTsfrom the liquid of the aqueous suspension of the CNTs. In thisembodiment, the CNTs are filtered onto the continuous porous carriermaterial to form the CNT structure and CNT sheet, and the CNT sheet isthen separated or peeled away from the continuous porous carriermaterial. In this embodiment, the CNT structure and CNT sheet willtypically have a basis weight of CNTs sufficient to provide the CNTstructure with integrity to be peeled away continuously from the carriermaterial as a free-standing CNT sheet, without tearing or degrading.

In a further aspect of the invention, a porous secondary layer canapplied to an upper side of the continuous porous carrier material,before applying the CNT suspension. The secondary layer can be acontinuous apertured or porous film, sheet or fabric material throughwhich the liquid of the aqueous suspension of the CNTs is drawn. Thesecondary layer, also referred to as a substrate layer or veil layer,when used over a carrier material, provides a continuous composite CNTsheet that has improved separation or “peel away” from the continuousporous carrier material. The secondary or veil layer can be very thinand light weight (low basis weight) to provide the separation of the CNTstructure from the carrier material, and can also improve or contributethe physical or function properties to the CNT structure and to thecomposite CNT sheet. The size of the openings (circular, square, or anyother shape) of candidate carrier materials are typically about a sizebetween about 0.1 micron, and up to about 5 mm. The length can be aspool or roll of material.

Thus, the present invention further includes a continuous process formanufacturing a continuous composite CNT sheet, comprising the steps of:i) moving a continuous conveying belt along a path that traverses apooling region and a vacuum box; ii) applying a continuous porouscarrier material to an upper side of the moving continuous conveyingbelt; iii) applying a continuous porous veil layer to an upper side ofthe continuous porous carrier material; iv) applying an aqueoussuspension of carbon nanotubes (CNTs) dispersed in a liquid on theporous veil layer, the dispersed CNTs; v) forming a continuous pool ofthe aqueous suspension of the CNTs over and across the width of theporous veil layer moving in the pooling region, the continuous pool ofthe aqueous suspension of the CNTs having a uniform thickness sufficientto prevent puddling upon the porous veil layer; vi) advancing the porousveil layer, porous carrier material, and the continuous pool of theaqueous suspension of the CNTs over the vacuum box; vii) drawing byvacuum the liquid of the aqueous suspension of the CNTs through theporous veil layer and the porous carrier material, and filtering auniform dispersion of filtered CNTs over the porous veil layer to form afiltered CNT structure; viii) optionally drying any residual liquid fromthe filtered CNT structure to form a composite CNT sheet over the porouscarrier material; and ix) removing the composite CNT sheet from thecontinuous porous carrier material. The CNTs comprise said entanglableCNTs.

In another aspect of the invention, the continuous porous carriermaterial can be removed from the process and replaced with a secondaryor veil layer, referred also to as a carrier-veil layer, that canprovide the function of the carrier material. The carrier-veil layerprovides filtration of CNTs from the CNT suspension, and has asufficiently stable and resilient porous structure sufficient forpulling the forming CNT structure through and along the apparatus duringprocessing, to prevent tearing and degradation of the CNT structureduring manufacture, while having physical and functional properties, forexample, low basis weight and minimal thickness, that are consistentwith the intended use of the CNT sheet. Optionally, in apost-manufacturing process, the CNT structure can be separated from theveil-carrier layer. Examples of suitable carriers include carbon fibernonwoven, polyester nonwoven, polyester woven, fiberglass nonwoven,expanded copper foil, copper mesh, or PEEK nonwoven. Optionally, thesesubstrates can be metallized to add functionally such as conductivity orother properties.

Thus, the present invention also includes a continuous process formanufacturing continuous CNT sheets, comprising the steps of: i) movinga continuous conveying belt along a path that traverses a pooling regionand a vacuum box; ii) applying a continuous porous carrier-veil layer toan upper side of the continuous conveying belt; iii) applying an aqueoussuspension of entanglable carbon nanotubes (CNTs) dispersed in a liquidon the porous carrier-veil layer; iv) forming a continuous pool of theaqueous suspension of the entanglable CNTs over and across the width ofthe continuous porous carrier-veil layer in the pooling region, thecontinuous pool of the aqueous suspension of the entanglable CNTs havinga uniform thickness sufficient to prevent puddling upon the continuousporous carrier-veil layer; v) advancing the continuous porouscarrier-veil layer and the continuous pool of the aqueous suspension ofthe entanglable CNTs over the vacuum box; vi) drawing by vacuum theliquid of the aqueous suspension of the entanglable CNTs through thecontinuous porous carrier-veil layer, and filtering a uniform dispersionof entanglable CNTs over the continuous porous carrier-veil layer toform an entangled CNT structure on the carrier-veil layer; vii)optionally drying any residual liquid from the entangled CNT structureto form a continuous composite CNT-veil sheet including the entangledCNT structure and the carrier-veil layer; and viii) removing thecontinuous composite CNT-veil sheet from the continuous conveying belt.

The present invention also provides a manufactured CNT sheet comprisingentanglable CNTs arranged in a randomly oriented, uniformly distributedpattern. The CNT sheet has a basis weight of at least 1 gram CNT persquare meter (gsm).

The manufactured CNT sheet has a relative density of about 1.5 or less(relative water).

The desired basis weight of the manufactured CNT structure can be atleast 2 gsm, at least 3 gsm, at least 4 gsm, at least 5 gsm, and atleast 6 gsm; and up to about 40 gsm, including up to about 30 gsm, up toabout 20 gsm, up to about 15 gsm, up to about 12 gsm, up to about 10gsm, up to about 8 gsm, and up to about 6 gsm; and can be about 3 gsm,about 4 gsm, about 5 gsm, about 6 gsm, about 7 gsm, about 8 gsm, about 9gsm, about 10 gsm, and about 15 gsm.

The relative density of the manufactured CNT structure can be about 1.0or less, and can be about 0.8 or less, about 0.7 or less, about 0.6 orless, about 0.5 or less, about 0.4 or less, and about 0.3 or less, suchas 0.25. Such relative densities are well below those of buckypapersdescribed in the art, and provide a CNT sheet with an effectivethickness with a substantially lower basis weight.

The entanglable CNTs useful in forming a filtered and/or entangled CNTstructure can include DWCNTs and MWCNTs having a median length of atleast 0.05 mm, or at least 0.1 mm, or at least 0.2 mm, or at least 0.3mm, a length of at least 0.4 mm, or at least 0.5 mm, and of at least 2mm, and can be highly elongated CNTs having an aspect ratio of at least2,500:1, including at least 5,000:1, at least 10,000:1, at least50,000:1, and at least 100,000:1. Such CNTs can also include single wallCNTs (SWCNTs) having a median length of at least 0.005 mm, or at least0.01 mm, or at least 0.02 mm, or at least 0.03 mm, a length of at least0.04 mm, and can be highly elongated SWCNTs having an aspect ratio of atleast 2,500:1, including at least 5,000:1, at least 10,000:1, at least50,000:1, at least 100,000:1, and at least 1,00,000:1.

The CNTs can also optionally include short MWCNTs (including DWCNTs thathave a median length of less than about 50 microns, and/or an aspectratio of less than 2,500:1.

The entanglable CNTs can also be combined with short MWCNTs to provideadditional functionality, typically in a weight ratio of entanglableCNTs to short MWCNTs selected from the group consisting of at least1:10, at least 1:3, at least 1:2, at least 1:1, at least 2:1, at least10:1, and at least 99:1.

In a further aspect of the invention, the CNT sheet can have afree-standing structure.

In another aspect of the invention, the mass proportion of theentanglable CNTs in the CNT sheet will comprise at least 75% of thetotal CNTs in the sheet or CNT structure, including at least 80%, atleast 85%, at least 90%, at least 95%, at least 97%, at least 98%, andat least 99% of the total CNTs in the sheet or CNT structure.

The CNT structure or sheet includes a nonwoven sheet comprising carbonCNTs that form a continuous matrix or solid phase across the entire areaof the nonwoven sheet. In this aspect, each CNT is in direct contactwith a plurality of adjacent CNTs along its length.

In a further aspect of the invention, the CNTs can be chemically treatedprior to sheet formation to modify the physical or functional propertiesof the CNTs, or of the nonwoven CNT sheet or structure made therefrom.

In another aspect of the invention, the CNTs can be pre-treated byimmersion into an acidic solution, including an organic or inorganicacid, and having a solution pH of less than 1.0. A non-limiting exampleof an acid is nitric acid. Alternatively, or in addition, the CNTstructure can be post-treated with an acid solution.

In another aspect of the invention, the fillers are added to the CNTsuspension to add functionality to the nonwoven structure. Thisincludes, but not limited to, adding conductive and/or non-conductivefillers such as carbon nanofiber, graphene, glass fiber, carbon fiber,thermoplastic fiber, thermoset fiber, glass microbubbles, glass powder,thermoplastic powder, thermoset powder, nickel nanowire, nickelnanostrands, chopped nickel coated carbon fiber, ceramic powder, ceramicfiber, or mixtures thereof, For example, nickel nanostrands can be addedto the CNT nonwoven to increase electrical conductivity andpermeability. These properties can increase EMI shielding properties.Another example includes adding multi-lobal polyimide fiber to the CNTnonwoven to improve mechanical properties in a carbon fiber compositesystem and adding multifunctionality to said composite system.

In another aspect of the invention, the CNTs nonwoven structure caninclude a plurality of distinctly formed CNT sheets, stacked orlaminated together. The stacked layers can also include filler oradditive materials. Example filler materials include, but are notlimited to, carbon nanofiber, graphene, glass fiber, carbon fiber,thermoplastic fiber, thermoset fiber, glass microbubbles, glass powder,thermoplastic powder, thermoset powder, nickel nanowire, nickelnanostrands, or mixtures thereof. For example, a solution containinggraphene can be laid onto and coupled to a previously formed CNTnonwoven layer using the herein mentioned continuous manufacturingprocess.

In yet another aspect of the invention, the aqueous liquid is drawnthrough the filter material with the aid of vacuum to pull water throughthe filter material, or with applied pressure to press water through thefilter material, or a combination thereof.

An apparatus for manufacturing a continuous CNT sheet is based on aconventional apparatus for making wet-laid nonwoven structures, andincludes a continuous conveying belt that moves along a path thattraverses a table including a pooling region and a vacuum box, anoptional drying unit, and various rollers and conveying elements forpulling the continuous conveying belt along the apparatus and throughthe process. The apparatus also includes the means for storing andsupplying a suspension of CNTs to the pooling region, and a headbox ormeans for forming a continuous pool of the aqueous suspension of theCNTs across the width of a filter material disposed on the continuousconveying belt in the pooling region.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a schematic of a conventional apparatus for making nonwovenwet-laid fabrics.

FIG. 2 shows apparatus that can be used in a process of the presentinvention for making CNT structures.

FIG. 3 shows the apparatus of FIG. 2 in which a solution of dispersedCNTs filling a reservoir and flowing in a flooding pool over acontinuous filter material.

FIG. 4 shows the apparatus of FIG. 3 after the flooding pool of solutionpasses over a vacuum box to form the CNT sheet structure.

FIG. 5 shows the apparatus and an alternative process of FIG. 3, furtherapplying a continuous porous veil layer over the continuous filtermaterial.

FIG. 6 shows the apparatus and another alternative process of FIG. 3,wherein the continuous filter material forms a continuous compositeCNT-filter material sheet.

FIG. 7 shows an alternative apparatus for forming the flooding pool ofthe solution of dispersed CNTs over the continuous filter material.

FIG. 8 shows another alternative apparatus using liquid spray nozzles toform the flooding pool of the solution of dispersed CNTs over thecontinuous filter material.

FIG. 9 shows a schematic of a CNT web-laying machine that includes twoor more headboxes for feeding additional and different suspensions ofCNTs or other materials.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the terms “comprise,” “comprising,” “include,” and“including” are intended to be open, non-limiting terms, unless thecontrary is expressly indicated.

As used herein, a “free-standing” sheet or structure of CNTs is one thatis capable of formation, or separation from a filter material, andhandling or manipulation without falling apart, or without significantflaking or crumbling of CNTs away from the sheet or structure.

A “continuous” sheet of material is an elongated sheet having a lengththat is orders of magnitude greater than the width of the sheet, and aroll of the sheet material.

Conventional wet-laid nonwovens are made by a modified papermakingprocess. That is, the fibers to be used are suspended in water or otherdispersive liquid. A specialized machine is used to separate the wateror other dispersive liquid from the fibers to form a uniform sheet ofmaterial, which is then dried. The wet-laid process has its origins inthe manufacture of paper and was developed because paper manufacturerswanted to be able to use uncut, long natural fibers and synthetic fibersin addition to the usual raw materials without changing the process.Nonwoven textile fibers tend to be longer, stronger, and relativelyinert when compared to papermaking fibers.

FIG. 1 shows a schematic of an apparatus for making nonwoven wet-laidfabrics. Three characteristic stages in the manufacture of nonwovenfabrics by the wet-laid method: 1) suspension of the fiber (1) in wateror other dispersive liquid, and transport of the suspension onto acontinuous traveling screen (12), 2) continuous web formation on thescreen as a result of filtration (20), and 3) drying (30) of the web.

A process for forming CNT structures of the present invention is animprovement on the conventional process for making wet-laid nonwovens. Aprocess for forming CNT structures includes a step or stage of forming asuspension of CNTs is a dispersive liquid, a filtering a volume of theCNT suspension to provide a uniform dispersion of a CNT structure overthe filter material, and drying any residual dispersive liquid from thefiltered CNT structure.

Making the Suspension

The first step in making a continuous length of CNT structure involvesmaking a suspension of CNTs in a dispersive liquid, which can includewater. The dispersive liquid can also include one or more compounds forimproving and stabilizing the dispersion and suspension of the CNTs inthe dispersive liquid, and one or more compounds that improve thefunctional properties of the CNT structure produced by the method.

While water is a preferred dispersive liquid, other non-solvatingliquids can be used to disperse and process the CNTs. As used herein,the term “non-solvating” refers to compounds in liquid form that arenon-reactive essentially with the CNTs and in which the CNTs areessentially insoluble. Examples of other suitable non-solvating liquidsinclude volatile organic liquids, selected from the group consisting ofacetone, ethanol, methanol, isopropanol, n-hexane, ether, acetonitrile,chloroform, DMF, THF (tetrahydrofuran), NMP (N-Methyl-2-pyrrolidone),MEK (methyl ethyl ketone), DMAC, and mixtures thereof. Low-boiling pointsolvents are typically preferred so that the solvent can be easily andquickly removed, facilitating drying of the resulting CNT structure.

The dispersive liquid can optionally include one or more surfactants(e.g., dispersant agents, anti-flocculants) to aid forming or tomaintain the dispersing, wet-laid formation, or dewatering of the CNTsand wet-laid CNT structures. For example, BYK-9076 (from BYK Chem USA),Triton X-100, dodecylbenzenesulfonic acid sodium salt (NaDDBS), and SDSmay be used.

The carbon nanotubes can be provided in a dry, bulk form. The CNTs caninclude entanglable CNTs that typically have a median length selectedfrom the group consisting of at least about 0.05 mm (50 microns), suchas at least about 0.1 mm (100 microns), at least about 0.2 mm, at leastabout 0.3 mm, at least about 0.4 mm, at least about 0.5 mm, at leastabout 1 mm, at least about 2 mm, and at least about 5 mm. The CNTs canbe said entanglable single wall nanotubes (SWNT), and said entanglablemulti-wall nanotubes (MWNT). Typical SWCNTs have a tube diameter ofabout 1 to 2 nanometers. Typical MWCNTs have a tube diameter of about 5to 10 nanometers. Examples of MWCNTs useful in the present invention arethose disclosed in or made by a process described in U.S. Pat. No.8,753,602, the disclosure of which is incorporated by reference in itsentirety. Such carbon nanotubes can include long, vertically-alignedCNTs, which are commercially available from General Nano LLC(Cincinnati, Ohio, USA). U.S. Pat. No. 8,137,653, the disclosure ofwhich is incorporated by reference in its entirety, discloses a methodof producing carbon nanotubes, and substantially single wall CNTs,comprising, in a reaction chamber, evaporating a partially meltedcatalyst electrode by an electrical arc discharge, condensing theevaporated catalyst vapors to form nanoparticles comprising thecatalyst, and decomposing gaseous hydrocarbons in the presence of thenanoparticles to form carbon nanotubes on the surface of the catalystnanoparticles.

A CNT concentration in the aqueous liquid is at least 100 mg/L ofsuspension, and up to about 10 g/L, which facilitates dispersion andsuspension, and minimizes agglomeration or flocculation of the CNTs inthe dispersing liquid. In various embodiments of the invention, the CNTconcentration is at least about 500 mg/L, and at least about 700 mg/L,and up to about 5 g/L, up to about 1 g/l, and up to about 500 mg/L.Further, the aqueous suspension can comprise a CNT level selected fromthe group consisting of about 1% CNTs by weight or less, about 0.5% CNTsby weight or less, about 0.1% CNTs by weight or less, about 0.07% CNTsby weight or less, about 0.05% CNTs by weight or less, and including atleast about 0.01% CNTs by weight, such as at least about 0.05% CNTs byweight.

Generally, the CNTs are added to a quantity of the dispersive liquidunder mixing conditions using one or more agitation or dispersingdevices known in the art. The CNT suspension can be made in a batchprocess or in a continuous process. In one embodiment, the mixture ofCNTs in the aqueous liquid is subjected to sonication using conventionalsonication equipment. The suspension of CNTs in water can also be formedusing high shear mixing, and microfluidic mixing techniques, describedin U.S. Pat. No. 8,283,403, the disclosure of which is incorporated byreference in its entirety. A non-limiting example of a high shear mixingdevice for dispersing CNTs in an aqueous liquid is a power injectionsystem, for either batch of in-line (continuous) mixing of CNT powderand the liquid, by injecting the powder into a high-shear rotor/statormixer, available as SLIM technology from Charles Ross & Sons Company.

The Power Number, N_(P), is commonly used as a dimensionless number formixing. It is defined as:N _(P) =P/(ω³ D ⁵ρ), where

-   -   P=power input of mixer,    -   ω=rotational speed of mixer,    -   D=mixing blade diameter, and    -   ρ=dispersion density of liquid

To compare mixing scale, we can analogize with the Kolmogorov scale ofmixing, λ, to the average length of CNTs, L.λ=(v ³/ε)^(1/4), where

-   -   v=kinematic viscosity of dispersion, and    -   ε=rate of dissipation of turbulence kinetic energy per unit        mass.

For entanglable CNTs, v is much higher (more viscous); thus, λ is largerbut scales to slightly less than linearly. But, it requires a lot ofenergy (to the 4^(th) power) to get to the same post mixing length.Also, note that ε should be about linear with P, the power input ofmixer.

Without being bound by any particular theory, it is believed that as aresult of mixing and dispersing entanglable CNTs in the aqueous liquid,the individual CNTs can reduce in length and the structure twisted orbent, and bundles of individual CNTs can become entangled and tied.Typically, the length of CNTs that are provided into the mixing anddispersing process are longer than those of the resulting dispersedCNTs; for example, a median length selected from the group consisting ofat least about 0.005 mm, and an aspect ratio of at least 2,500:1. Themedian length can be at least 0.1 mm, at least about 0.2 mm, at leastabout 0.3 mm, at least about 0.4 mm, at least about 0.5 mm, at leastabout 1 mm, at least about 2 mm, and at least about 5 mm. The medianlength of the CNTs can also comprise a range selected from the groupconsisting of between 1 mm and 2 mm, between 1 mm and 3 mm, and between2 mm and 3 mm. The aspect ratio can be at least 5,000:1, at least10,000:1, at least 50,000:1, and at least 100,000:1,

The resulting suspension of CNTs in the aqueous liquid is stable for atleast several days, and longer. The suspension of CNTs can be mixed andstirred prior to use in the sheet forming process in order to ensurehomogeneity of the CNT dispersion.

In an aspect of the invention, the entanglable CNTs can be used with theshort MWCNTs in a weight ratio selected from the group consisting of atleast 1:99, at least 1:10, at least 1:3, at least 1:2, at least 1:1, andat least 2:1. The combination can include a combination of thefunctional properties and features of each type of CNT, includingsynergistic functional properties and features.

The dispersive liquid can also optionally include one or more bindercompounds for improve both the structural and functional properties ofthe CNTs. An example of a binder is polyvinyl acrylate (PVA). The use ofa binder such as PVA can improve the structure and function of CNTnonwoven sheets comprising both the entanglable CNTs and the shortMWCNTs. The binder is also useful for improving adherence of thedispersed CNTs into a hydrophobic filter material, such as Teflon®. In afurther aspect of the invention, the addition of the binder can resultin a continuous phase of the binder, which reduces the contact points(or CNT to CNT junctions) within the CNT sheet. The binder coats orencapsulates the CNTs within the polymer or resin matrix, separating theadjacent CNTs in close proximity by a layer of the binder material. Thebinder results in at least a partial disruption of the continuous matrixof the CNT sheet.

In one example, CNTs, as described above, are acid treated in fumingnitric acid, and then dispersed mechanically into the dispersive liquid,for example, water (without any surfactant). After storage, thedispersion of CNTs in the solution persisted at least 5 calendar days.Two 55-gallon drums of the CNT suspension were discharged into a200-gallon make-up tank, and low intensity agitation was applied priorto laying down of the suspension onto the sheet filter material. Withinjust 5 minutes, a 10-gsm nonwoven structure was formed with gooddispersion and uniformity of the CNTs within the nonwoven structure. Incomparison to a suspension of short MWCNTs, the dispersion andsuspension stability of the entanglable CNTs is surprisingly very longin time. Without being bound by any particular theory, it is believedthat the relative ease of formation of the CNT suspension is heavilyinfluenced by the aspect ratio of the CNTs (that is, the length of anindividual tube or fiber, to its mean diameter). The entanglable CNTshave a suspension stability that is at least an order of magnitudelonger than short MWCNTs. The use of CNTs having the prescribed aspectratio enabled the formation of nonwoven CNT sheets having a basis weightas low as 2 gsm. It has been determined that nonwovens made from woodpulp fibers having a significantly smaller aspect ratio (about 100:1)could not be formed on a continuous, wet-laid nonwoven apparatus at abasis weight at or below of 10 gsm.

The high aspect ratios of the entanglable CNTs also result in a CNTnonwoven having a critical entanglement density well in excess of 100,where the critical entanglement density is defined as the ratio of theaverage length of a CNT to the average length between the two adjacentintersections of the CNT with other CNTs.

The dispersive liquid can also optionally include one or more filler orfunctional filler materials. A functional filler material can be onethat has properties that may modulate the properties of the CNT sheet orstructure that is produced by the process described herein. Suchfunction fillers (or properties) can include non-magnetic dielectricmaterials, magnetic dielectric materials, electrically non-conductivematerials, electrically conductive materials. The materials can includeparticles, agglomerates, fibers, and others. Examples of non-magneticdielectric materials include epoxies, polyamides, and polyimides.Examples of magnetic dielectric materials include ferrite,ferrite-filled epoxy, ferrite-filled polyimide, and ferrite-filledpolyamide. Examples of electrically non-conductive materials includethermoplastic or thermoset materials, including without limitation,polyamide, polyimide, round or multi-lobal thermoplastic fibers, andpolyamide and polyimide thermoset powder. Other examples of electricallynon-conductive materials include ceramic fibers, including by examplealumina, boron nitride, ceramic powder, including by example aluminaboron nitride, ferrites including Fe₂O₃ and Fe₃O₄, MnZn, NiZn, andnanoparticles including graphene and gold nanoparticles. Examples ofelectrically conductive materials include metal nanofibers or wireincluding by example nickel nano-strands and silver nanowire, metalizedfibers including by example chopped nickel coated carbon fiber, andnanoparticles including by example graphene and gold nanoparticles.

Filtration and CNT Structure Formation

The second step in making the CNT structure comprises passing a volumeof the CNT suspension over a filter material, and drawing the dispersiveliquid of the CNT suspension through the filter material to provide auniform dispersion of the CNTs over the filter material. The CNTsuspension can be pulled or pushed through the filter material undervacuum or pressure.

The filter material is a flexible, resilient sheet material having poresor openings that are sufficiently large to allow the dispersive liquidto be drawn through with a moderate amount of vacuum or pressure, thoughare sufficiently small to prevent the multitude of dispersed CNTs frompassing through. The size of the openings (circular, square or any othershape) are typically about a size between about 0.1 micron, and up toabout 10 micron, and the porosity (open area) is typically about 20% toabout 80%, and selected from the group consisting of about 30%, about40% or about 50%.

The loading (weight per area) of the dispersed CNTs in the aqueousliquid onto the surface of the filter material can be determined fromthe desired basis weight of the resulting CNT structure (in grams persquare meter, or “gsm”), and the concentration of dispersed CNTs in thedispersive liquid. The solution comprising the dispersed CNTs is loadedover a mesh screen, and in a manufacturing process, a continuous belt ormesh screen. A continuous layer of filter material is registered withand passes along the outer surface of the mesh screen, and serves as aretaining filter for the CNTs. The filter material is preferablynon-soluble in and non-absorbent of water or of the dispersive liquid,and can include both hydrophilic materials, including nylon, andhydrophobic materials, including Teflon®. Hydrophilic or hydrophobiccoatings, as applicable, can also be applied to a base structure of thefilter material. The filter material is also referred to herein as ascrim. The filter material can include a continuous web of material,ranging from 10 inches (25 cm), and up to 60 inches (152 cm) in width,and of continuous lineal length. The length can be a spool or roll ofmaterial, or a continuous loop of material, depending upon whether theresulting CNT nonwoven sheet is removed from the scrim continuouslyfollowing drying at the same production site (mentioned herein after),or is processed remotely.

CNTs inherently are poorly dispersible in aqueous solutions, even afterbeing treated to provide improved hydrophilicity. Under some dispersionconditions, the CNTs, and particularly CNTs having a long aspect ratio,tend to flocculate or aggregate into bundles or larger clot-like lumpsin the aqueous dispersion. The conditions of the continuous processtherefore inhibit or prevent puddling of the CNT suspension and manageand control the inherent tendency of dispersed CNTs to flocculate oragglomerate.

The loaded filter material is then passed over a vacuum zone or vacuumbox, which draws the dispersing liquid away from the dispersed andentangled CNTs, and through the openings in the filter material. Themesh screen or belt is typically a stainless steel, and has a pattern ofwire mesh sufficient to support the filter material in a plane as itpasses over the vacuum zone (also called a vacuum box). The strength ormagnitude of the vacuum (pressure) and the length of the vacuum zone (ordwell time) are sufficient to draw substantially all of the freedispersive solution from above the filter material, while also allowingthe dispersed CNTs to settle onto the filter material, typically in arandomly-oriented, uniformly-distributed pattern upon the filtermaterial. Uniformly distributed CNTs will appear as a uniform, blackmaterial surface across the entire width of the filter material.Typically the CNT sheet structure has a uniformity of not more than 10%coefficient of variance (COV), wherein COV is determined by awell-known, conventional method. In an aspect of the invention, thecarbon nanotubes (CNTs) comprised in the nonwoven sheet form acontinuous matrix or phase across the entire area of the nonwoven sheet,where the CNTs are in direct contact with one or more adjacent CNTsalong their lengths. Selection of vacuum box dimensions of length andwidth, can be optimized as needed for different CNT lengths and CNTnonwoven sheet bases weights. The CNT structure can become attached tothe filtering material through the CNT material at the surface, forminga CNT/filtering material composite.

The desired basis weight of the resulting CNT structure is affected byseveral parameters, including process conditions, apparatus, and thematerials used. Generally, the larger the basis weight required, thehigher the required CNT concentration, and/or the larger the dispersedliquid loading, and/or the larger the vacuum zone area, and/or thehigher the vacuum applied, and/or the slower the linear speed of thefilter material over the vacuum zone. All of these parameters can bemanipulated to achieve specific desired characteristics of the CNTnonwoven sheet, including its thickness, density, and porosity.

CNT Nonwoven Sheets

The CNT nonwoven sheets made according to the present invention, whenused alone or as part of a composite structure or laminate, can providenumerous mechanical and functional benefits and properties, includingelectrical properties. The CNT nonwoven sheets and composite laminatesand structures thereof can be used for constructing long and continuousthermal and electrical paths using CNTs in large structures or devices.The CNT nonwoven sheets and composite laminates and structures thereofcan be used in a very wide variety of products and technologies,including aerospace, communications and power wire and cable, windenergy apparatus, sporting goods, etc. The CNT nonwoven sheets andcomposite laminates and structures thereof are useful as light-weightmultifunctional composite structures that have high strength andelectrical conductivity. The CNT nonwoven sheets and composite laminatesand structures thereof can be provided in roll stock of any desirableand commercially-useful width, which can integrate into mostconventional product manufacturing systems.

Non-limiting examples of functional properties, and the modulationthereof, that can be provided by the CNT nonwoven sheets and compositelaminates and structures thereof, are conductive composites,electromagnetic wave absorption, in-situ structural health monitoring,lightning strike prevention and dissipation, water filtration,electromagnetic interference (EMI) shielding, thermal interface pads,energy storage, supercapacitor, and heat dissipation.

The desired basis weight of the resulting CNT structure is at least 1grams of the CNTs per square meter (gsm), which can include a CNT basisweight selected from the group consisting of at least 1 gsm, at least 2gsm, at least 3 gsm, at least 4 gsm, at least 5 gsm, and at least 6 gsm;and up to about 40 gsm, including up to about 30 gsm, up to about 20gsm, up to about 15 gsm, up to about 12 gsm, up to about 10 gsm, up toabout 8 gsm, and up to about 6 gsm; and can be about 3 gsm, about 4 gsm,about 5 gsm, about 6 gsm, about 7 gsm, about 8 gsm, about 9 gsm, about10 gsm, and about 15 gsm.

CNT nonwoven substrates having very low basis weight, typically of about4 gsm or less, are so thin that they cannot be separated themselves,independently, from the filter material (scrim) without falling apart.CNT nonwoven sheets having very low basis weight can be separated fromthe filter material using a tacky substrate that itself comprises amember of a composite structure.

The present invention also includes a secondary web material that isprocessed with the CNT suspension liquid. The secondary web material caninclude low basis weight fiberglass, melt-spun or wet-laid nonwovensmade of thermoplastics, including polyester, and carbon fiber veils orwebs. In one embodiment of the invention, the secondary web material canbe the filter material onto which the CNT suspension is applied andthrough which the dispersive liquid is drawn to deposit the CNT nonwovensheet. In another embodiment, the secondary web material can be disposedin registry upon the upper surface of the filter material to form a dualfilter material. Typically, after drying of the CNT nonwoven sheet, thesecondary web material is removed from the base filter material with theCNTs attached thereto. The use of a secondary web material is alsoadvantageous with very low bases weight CNT nonwoven sheets, asdescribed above.

Functionalizing of CNTs

Functional properties of a CNT nonwoven sheet can be affected bytreatment of the CNTs of the CNT nonwoven sheet, prior to theirdispersion and suspension, or after formation into a CNT nonwoven sheet.The treatment of the CNTs or of the CNT nonwoven sheet can include achemical treatment or a mechanical treatment.

In one aspect of the invention, functional properties of a CNT nonwovensheet can be affected by an acid treatment of the CNTs, prior to theirdispersion and suspension, or by a post-formation acid treatment. Thepost-formation treatment can be performed either in a batch treatmentprocess, or in a continuous (roll) process, by immersing the CNTnonwoven sheet into an acid bath or by application of an acid solutionthereto, followed by rinsing with water to remove residual acid, anddrying. An acid treatment is believed to improve CNT purity and quality,by reducing the level of amorphous carbon and other defects in the CNTs.Treatment of the bulk CNT powder with strong (nitric) acid can causeend-cap cutting, and the introduction of carboxyl groups to the CNTsidewall. The addition of carboxyl groups to the CNT sidewalls can alsoenhance dispersion of the CNTs in water or other polar solvent byincreasing the hydrophilicity of the CNTs. The removal of amorphouscarbon coatings on individual nanotubes increases the concentration ofcrosslink joints and higher bending modulus, which can create moreconductive tunnels and connections. CNT end-cap cutting can improveelectrical conductivity by improving electron mobility from the ends ofthe carbon nanotubes to adjacent carbon nanotubes (tunneling). Likewise,post-formation acid treatment can improve electrical conductivity andincrease the structure's density.

The acid treatment of the CNTs enhances CNT interactions andcharge-carrying and transport capabilities. Acid treatment of the CNTscan also enhance cross-linking with a polymer composite. Without beingbound by any particular theory, it is believed that during acidoxidation, the carbon-carbon bonded network of the graphitic layers isbroken, allowing the introduction of oxygen units in the form ofcarboxyl, phenolic and lactone groups, which have been extensivelyexploited for further chemical functionalization.

The pre-treatment of the CNTs can include immersing the CNTs into anacidic solution. The acid solution can be a concentrated or fumingsolution. The acid can be selected from an organic acid or inorganicacid, and can include an acid that provides a solution pH of less than1.0. Examples of an acid are nitric acid, sulfuric acid, and mixtures orcombinations thereof. In an embodiment of the invention, the acid is a3:1 (mass) ratio of nitric and sulfuric acid.

Alternatively or in addition to acid post-post treatment, the CNT powderor formed CNT nonwoven sheet can be functionalized with lowpressure/atmospheric pressure plasma, as described in NanotubeSuperfiber Materials, Chapter 13, Malik et al, (2014), the disclosure ofwhich is incorporated by reference in its entirety. A Surfx Atomflo400-D reactor employing oxygen and helium as the active and carriergases, respectively, provides a suitable bench-scale device for plasmafunctionalizing CNTs and CNT non-woven sheet structures. An alternativeplasma device can include a linear plasma head for continuousfunctionalization of CNT non-woven sheets, including non-woven rollstock. An atmospheric plasma device produces an oxygen plasma stream atlow temperature, which minimizes or prevents damage to the CNTs and theCNT structures. In an example, a plasma is formed by feeding He at aconstant flow rate of 30 L/min and the flow rate of O₂ (0.2-0.65 L/min)is adjusted as per the plasma power desired. Structural and chemicalmodifications induced by plasma treatments on the MWCNTs can be tailoredto promote adhesion or to modify other mechanical or electricalproperties. Additionally, plasma functionalization can be used to cleanthe surface of the CNT nonwoven structure, cross-link surface molecules,or even generate other polar groups on the surface to which additionalfunctional groups can be attached. The extent to which the CNT nonwovenare affected by plasma functionalization can be characterized usingRaman spectroscopy, XPS, FTIR spectroscopy and changes in hydrophobiccharacter of the CNT material through contact angle testing.

Another example of a chemical treatment is the addition of a largemolecule onto the CNT structure which reduces the CNT-to-CNT contactsalong the length of the carbon nanotubes. An example of a large moleculeis an epoxide. Epoxide functionalization has been shown to increase theelectrical resistance of a non-woven sheet. Further increases in sheetresistance have been shown by adding surfactants to the epoxidefunctionalized CNT dispersion which act to further separate the distancebetween adjacent carbon nanotubes. A 20× increase in sheet resistancehas been observed with this approach, increasing the sheet resistancefrom about 5 ohms per square (Ω/□) to about 100Ω/□ for a 10 gsmnonwoven.

Another example of a chemical treatment is the treatment of the CNTswith fluorine. The bulk CNT powder or CNT sheets or structures can betreated with fluorine gas. Fluorination can increase resistance of CNTsto the point of becoming electrical insulators. The process usesfluorine gas at temperatures above 250° C. to create C—F bonds on thesidewalls of each individual carbon nanotubes.

Conducting or non-conductive particles or fibers can also be added tothe CNTs to enhance or suppress electrical conductivity. In anon-limiting example, the addition of fiberglass flock to an entanglableCNT dispersion increased the electrical resistance and mechanicalproperties of a sheet. For example, the electrical resistance of a 1.0gsm CNT sheet (CNT basis only) is increased from about 50Ω/□ to about60Ω/□, an increase of 20%, while increasing the composite overall basisweight to 26.0 gsm. Of note, graphene and carbon nanofibers are used asfiller in the high sheet resistance blends (on polyester) to maintainhigh uniformity at such low basis weights (about 2 gsm).

The present invention also includes producing a low basis weight (2 gsm)nonwoven sheet using either of entanglable CNTs or short MWCNTs, or bothentanglable CNTs and short MWCNTs, on the wet-laying nonwoven apparatusthat can include modifications in the construction and operation asdescribed herein.

Electrical conductivity within the nonwoven CNT sheet structure can beenhanced or suppressed by mechanical treatment and processing of the CNTsheet. First, and simply, the basis weight can be increased to decreasesheet resistance (increase conductivity, at the cost and result ofadditional sheet weight and thickness). Empirically, the sheetresistance was found to “bottom out” at about 0.2Ω/□ with increasedbasis weight for a pretreated CNT sheet.

Mechanical methods for increasing electrical conductance (decreasedsheet resistance) of a layer or nonwoven sheet of CNT include a methodresulting in improved alignment of CNTs within the sheet, anddensification of the CNT layer within the sheet. In addition, animproved dispersion of CNTs in the aqueous dispersion provides moredensely packed CNTs in the non-woven CNT sheet, and the denser CNTpacking in the nonwoven sheet results in a more conductive sheet anddifferent mechanical or physical properties. Another approach beinginvestigated is oxidation of the CNT sidewall via microwave excitation.This approach is similar to acid treatment in that it allows for betterdispersion, which results in a denser nonwoven.

In another aspect of the invention, functional properties of a CNTnonwoven sheet can be affected by metallization of the CNTs, prior totheir dispersion and suspension, or by metallization post-formation. Thepost-formation treatment can be performed either in a batch treatmentprocess, or in a continuous (roll) process, by such methods as physicaldeposition (e.g. sputtering, physical vapor deposition (PVD), pulsedlaser deposition (PLD), electron beam), chemical vapor deposition,electro-chemical (electroplating), or electroless coating. For example,a continuous nickel CVD process can be used to add a layer of nickel tothe surface of the CNT sheet material, thereby increasing conductivityof the CNT nonwoven.

Manufacturing Process for a Continuous Sheet of CNT Structure

An apparatus 110 of the present invention useful in a manufacturingprocess 100 for forming CNT structures is shown in FIGS. 2-4. CNTs aredispersed by mixing in a water or solvent solution and are contained ina mixing/storage tank 101. The CNT suspension 104 contained in themixing/storage tank 101 can be prepared remotely and transported to thefacility for manufacturing the CNT structures, or can be prepared at themanufacturing site. The second step in making the CNT structurecomprises passing a volume of the CNT suspension over a filter material,and drawing the dispersive liquid of the CNT suspension through thefilter material to provide a uniform dispersion of the CNTs over thefilter material. The CNT suspension 104 of CNTs in water is delivered bypump 102, or alternatively poured by gravity, at a controlled volumetricrate over a continuous porous belt 130, typically through a continuoussheet filter material or scrim 132.

A continuous porous belt 130 is typically a metallic, plastic-coatedmetallic, thermoplastic, or composite conveying belt, mesh or screen.The continuous porous belt 130 can have hinged sections or segments thatprovide industrial durability and reliable processing of the beltthrough the wet-laid apparatus 110, drier 180, and through the rollers136, including for separation of the final CNT structure from the filtermaterial, and its storage onto a product roller 192. The conventionalconstruction of the porous belt 130 in a wet-laid nonwoven processapparatus provides very poor filtration of the nano-sized CNTs, whichreadily pour through the openings in the continuous belt 130, thisindicating the need for a separate porous filter medium. The continuousporous belt 130 should have low extensibility or stretch in the machinedirection for pulling the porous belt through the pathway of theapparatus. In general, the porous belt 130 should not be used as afilter medium for filtering CNTs from the CNT suspension, because thedirectly-deposited CNT structure may be difficult to separate from theporous belt, particularly in an industrial or commercial process.

The continuous porous belt 130 passes in a continuous loop or belt,carrying a filter medium, shown as a continuous porous carrier material132, across and through the apparatus 110.

The continuous porous carrier material is a flexible, resilient sheetmaterial having pores or openings that are sufficiently large to allowthe dispersive liquid to be drawn through with a moderate amount ofvacuum or pressure, though are sufficiently small to prevent themultitude of dispersed CNTs from passing through when the carriermaterial is also used as a filter material. The size of the openings(circular, square or any other shape) are typically about a size betweenabout 0.1 micron, and up to about 10 micron, and the porosity (openarea) is typically about 200% to about 80%, and selected from the groupconsisting of about 30%, about 40% or about 50%.

A continuous porous carrier material (generally referred to as a scrim)132 provides a stable structure for pulling the forming CNT structurethrough and along the apparatus 110, to prevent tearing and degradationduring manufacturing. The continuous porous carrier material has a widthup to about 152 cm (60 inches). The scrim is typically unrolled from asupply roller 138 and applied to the upper surface of the moving,continuous porous belt 130, and after removal of the CNT sheet product,is separated from the continuous porous belt 130 and re-rolled onto areuse roller 139, which is then reused as a supply roller 138. Anotherfunction of the continuous porous carrier material 132 is to provide amore planar, smoother filtration surface, as compared to the continuousporous belt 130, which provides improved planar uniformity to theresulting CNT structure.

The continuous filter material 132 can provide the primary filtration ofthe CNT material from the dispersing liquid, and in this situation canbe referred to as a filtering scrim. The filtering scrim 132 cancomprise either a hydrophobic material or a hydrophilic material. Asuitable hydrophobic material is a high porosity Teflon™ mesh. Thehydrophobicity provides suitable and satisfactory release of theresulting CNT structure from the Teflon™ mesh scrim. A suitablehydrophilic material is a high porosity nylon mesh.

The apparatus 110 includes a table 111 having a planar top surface 112,along the length of which is drawn the continuous porous belt 130, froman inlet end 114 to outlet end 116. In a center portion of the planartop surface 112 of the apparatus 110 are a plurality of vacuum slots 120formed in series along the length and extending transversely across thewidth of the top surface 112. The vacuum slots 120 are disposed fordrawing the dispersing liquid away from the filtered CNTs under vacuum124. The vacuum capacity through each of the plurality of vacuum slots120 can be independently controlled 122 for improved deposition andpinning of the CNTs onto the continuous filter scrim 132, along the pathtraversing the vacuum box. The vacuum applied along the one or morevacuum slots 120 can be controlled independently. The typical vacuum isat least −1.0 psig, including in a range from about −2.5 psi to about−14 psi, and a typical vacuum gradient is about 2 psi to about 11 psig,along the length (machine direction) of the vacuum box, with a strongervacuum applied at the leading-most vacuum slots, such as 120 a. Theplurality of slots 120 can closely spaced apart, a single enlargedvacuum opening can be used, to provide a substantially continuous vacuumarea. Alternatively, the plurality of slots can be spaced apart withintermediate surfaces therebetween without a vacuum force upon theremaining flooding pool of CNT or the filtering CNT structure thatpasses over.

The physical length of the vacuum box of the present apparatus can beexpected to be at least 5-10 times the length the vacuum box in aconventional wet-laid nonwoven process. The vacuum residence time underwhich the flooding pool of CNT solution is exposed to the vacuumfiltration can be up to about 1 minute, including up to about 10seconds, and up to about 1 second.

The manufacturing line speed of the apparatus and method of the presentinvention is at least about 1 foot per minute (fpm), including at leastabout 10 fpm, at least 50 fpm, and up to about 100 fpm and more.

The hydrophobicity and porosity of a filter scrim 132, for example, aTeflon™ mesh scrim material, can result in puddling of the solution ofCNT suspension placed upon the advancing flat surface of the filterscrim 132. Puddling occurs when the solution of the aqueous CNTdispersion (a hydrophilic solution with dispersed CNTs) formsdiscontinuous and separated puddles on the hydrophobic filter scrim 132,typically due to their difference in interfacial surface tension. Theapplication of any vacuum force on the underside of a filter scrim onwhich puddling of the solution of CNT suspension has occurred, resultsin the CNTs within the puddles forming into discontinuous patches of thefiltered CNTs.

Another factor that can affect CNT filtration and the uniformity of theCNT structure is the dispersibility of the CNTs in an aqueous solution.CNTs inherently are poorly dispersible in aqueous solutions, even afterbeing treated to provide improved hydrophilicity. Even after dispersion,the CNTs, and particularly entanglable CNTs having a long aspect ratio,tend to flocculate or aggregate into bundles or larger clot-like lumpsin the aqueous dispersion. The conditions of the continuous process canbe managed to inhibit or prevent puddling of the CNT suspension, and tocontrol the inherent tendency of dispersed CNTs to flocculate oragglomerate.

In an embodiment of the invention, the apparatus 110 includes areservoir or “headbox” 140 at the inlet end 114 that is bounded by thewalls of the apparatus on three sides, and by a positionable sluice gate142 on the downstream side that allows a portion of the CNT dispersionvolume 144 within the reservoir 140 to flow under the bottom edge ofsluice gate 142. The spacing under the sluice gate 142 is sufficient topass thereunder the continuous porous belt 130 with the filter scrim132, and to permit a controlled amount and depth of the CNT suspensionto flow out of the reservoir. Downstream 118 from the sluice gate 142 isa flooded pool 146 of the CNT suspension that flows in the direction ofmovement of the scrim 132. The velocity v2 of the mass of the floodedpool 146 in the pooling area 118, flowing toward the vacuum slots 120,is two or more times, and up to 5 times, and up to 10 times, the linearvelocity of the moving scrim 132. The depth of the flooded pool 146across the width of the apparatus 110 is uniform and sufficient tospread and cover the entire surface of the filter scrim 132, to preventpuddling of the solution before arriving at the vacuum slots 120.

As the advancing flooded pool 146 above the scrim 132 arrives at thefirst of the vacuum slots 120 a, the liquid within the CNT dispersion isstrongly drawn through the filter scrim 132, which pins and sets a firstportion or layer of the CNTs upon the filter scrim 132 (FIG. 3). As theflooded pool 146 above the filter scrim 132 advances and arrives at thesecond and subsequent vacuum slots 120, addition portions of layers ofCNTs are pinned to and deposited upon the upper surface of the filterscrim 132. Without being bound by any particular theory, it is believedthat providing a uniform layer of filtered CNTs onto the upper surfaceof the filter scrim 132 (or porous filter material, as well as anysecondary veil-filter layer, described hereinafter) modifies theinterfacial surface of the filter scrim 132 and improves the wetting ofits surface by the CNT suspension, thereby minimizing and eliminatingpuddling upon the effected surface of the filter scrim.

As the advancing flooded pool 146 advances further with the filter scrim132, its velocity v2 slows as more of the liquid is drawn through thefilter scrim 132 by the successive vacuum slots 20. This also increasesthe density and thickness of the CNTs above and along the filter scrim132. The slowing of the velocity of the flooded pool 146 permits anyflocculated bundles or lumps of the CNTs in the upstream flooded pool146 to “pack in” behind the depositing CNT structure, thus improvinguniformity and reducing the variability in the density and thickness ofthe CNTs. Eventually, the forward edge of the flooded pool 146 “driesup” when a sufficient amount of the dispersing liquid has been drawnthrough, depositing all of the CNTs onto the filter scrim 132.Substantially all of the liquid from the CNT dispersion is drawn throughthe filter scrim 132 as the CNT structure 150 exits the last vacuum slot120 f.

After drawing away most of the water or other dispersive liquid, theadvancing continuous CNT structure 150 is dried to a CNT sheet using aconvection, contact and radiation dryer 180 (FIG. 4). The dried,continuous CNT sheet 155 is separated from the filter scrim 132 andtaken up on a product roller 158. The separated scrim 132 is taken up ona reuse roller 139 for reuse in the process as scrim 132.

FIGS. 7 and 8 show alternative apparatus and methods for providing acontrolled amount and depth of the CNT suspension in a flooded pool 146that flows over and in the direction of movement of the filter scrim132. FIG. 7 shows a container 242 including walls sufficient to containa depth of CNT suspension 244, and a base 272 having apertures 274. Thecontainer 242 extends across the width of the filter scrim 132 todistribute the CNT solution flowing through the apertures 274 within thepooling area 118. The apertures 274 are sized in area and provided innumber to maintain a depth of the flooded pool 146 sufficient to preventpuddling of the CNT suspension upon the surface of the filter scrim 132.FIG. 8 shows a spray distribution device 342 including a manifold 373and a plurality of spray nozzles 374. The plurality of nozzles 374extend across the width of the filter scrim 132 to distribute the CNTsolution flowing through the nozzles 374 within the pooling area 118.The nozzle size, number and distribution likewise maintains a depth ofthe flooded pool 146 sufficient to prevent puddling of the CNTsuspension upon the surface of the filter scrim 132.

The solution of dispersed CNTs can be distributed or applied onto thefiltering surface by other well-known methods, including spraying,rolling, coating, and casting.

FIG. 5 shows the apparatus 110 in a similar manufacturing process forforming CNT structures wherein a secondary veil layer 192 is unrolledfrom a veil supply roller 190 and applied continuously onto the movingupper surface of the scrim 132 at roller 196 to form a veil-carrierlaminate 194. The veil-carrier laminate 194 is then continuously appliedonto the belt 130 are previously described, and passed through and alongthe table, headbox(es) and vacuum slots apparatus 110. The CNTsuspension is applied, flooded across, and filtered through theveil-carrier laminate 194, substantially as described above, to form acontinuous CNT structure 150 on the veil-carrier laminate 194. In thisembodiment, the veil layer 192 can be the primary filter medium, withthe CNTs filtered directly onto the upper surface and elements of theveil layer, or the scrim (filter scrim) can be the primary filtermedium, where an initial layer of CNTs can pass through the veil layermaterial and pinned and filtered on the filter scrim 132, and theremaining filtered CNTs layering on and above the upper surface of theveil layer; or a combination thereof. After forming and drying thecontinuous CNT structure 150, a composite layer 156 of the CNT structure150 and the veil layer 192 can be separated from the carrier material132, and stored on a product roller 158. The carrier material 132 istaken up on a reuse roller 139 for reuse in the process as scrim 132.

FIG. 6 shows the apparatus 110 in a further similar manufacturingprocess for forming CNT structures wherein a continuous porouscarrier-veil layer 198 is unrolled from a carrier-veil supply roller 197and applied continuously onto the moving upper side of the continuousconveying belt 130, The carrier-veil layer 198 is then passed throughand along the table, headbox(es) and vacuum slots apparatus 110. The CNTsuspension is applied, flooded across, and filtered through thecarrier-veil layer 198, substantially as described above, to form acontinuous CNT structure 150 onto the carrier-veil layer 198. In thisembodiment, the carrier-veil layer 198 is the primary filter medium forthe CNT suspension. After forming and drying the continuous CNTstructure 150, a composite unitary layer 157 of the CNT structure 150and the carrier-veil layer 198 can be separated from the belt 130, andstored on a product roller 158.

The resulting CNT sheet made with the above manufacturing processes canhave a low density and low basis weight CNT structure with theuniformity and integrity for a wide variety of technology and industrialuses. The process also provides the flexibility to form effective CNTsheets from either SWCNTs or MWCNTs, or a combination thereof. Forexample, using a SWCNT having a median length above 50 microns and anaspect ratio greater than 5,000, a CNT sheet having a thickness of 40microns and a relative density of 0.5 provides a basis weight of about20 gsm, while a MWCNT having a median length above 100 microns and anaspect ratio greater than 5,000 forms CNT sheet having a thickness of 40microns and a relative density of 0.25, with a basis weight of about 10gsm,

The desired basis weight of the manufactured CNT structure can be atleast 1 gsm, at least 3 gsm, at least 4 gsm, at least 5 gsm, and atleast 6 gsm; and up to about 40 gsm, including up to about 30 gsm, up toabout 20 gsm, up to about 15 gsm, up to about 12 gsm, up to about 10gsm, up to about 8 gsm, and up to about 6 gsm; and can be about 3 gsm,about 4 gsm, about 5 gsm, about 6 gsm, about 7 gsm, about 8 gsm, about 9gsm, about 10 gsm, and about 15 gsm.

The relative density of the manufactured CNT structure can be about 1.0or less, and can be about 0.8 or less, about 0.7 or less, about 0.6 orless, about 0.5 or less, about 0.4 or less, and about 0.3 or less, suchas 0.25. Such relative densities are well below those of buckypapersdescribed in the art, and provide a CNT sheet with good uniformity andstructural stability, an acceptable thickness, and a substantially lowerbasis weight.

Wet-laid CNT sheet can also employ bonding agents, including water orsolvent-based crosslinkable synthetic polymer to provide flexibility andstrength.

A secondary layer as described above can be used to support to the CNTstructure, forming a composite sheet product to improve the mechanicalproperties and handling (processability) of the CNT sheet. The secondarylayer, also referred to herein as a substrate or veil layer in thecomposite sheet, is typically a fabric, woven, or non-woven material,and is typically porous and flexible. In an embodiment of the invention,the veil layer is non-stretchable or non-extensive, to limit or presentstretching of the composite sheet product. The veil layer can beprocessed with the CNTs in the wet-laid process, through and onto whichthe CNTs and any blended adjunct components can be vacuum formed, or canbe post-laminated or made co-extensive with the formed CNT sheets.Examples of a suitable veil layers can include, but not limited to,carbon fiber nonwoven, polyester woven or nonwoven, fiberglass nonwovenor woven, expanded copper foils, meshes, screens, or PEEK (polyetherether ketone) nonwoven.

In another embodiment of the invention, a wet-laying nonwoven apparatuscan include a plurality of headboxes for depositing a second dispersedCNT solution of suspended CNTs, fibers or other materials over the meshscreen. FIG. 9 shows a schematic of the wet-laying nonwoven apparatus300 that includes two or more reservoirs or head boxes (illustrated ashead boxes 140, 240, and 340), and a corresponding two or more tableswith vacuum boxes (illustrated as tables 111, 211, and 311), arranged inseries along the length of the continuous belt 130. The suspended CNTs,fibers or materials of the second dispersed CNT solution can include asecond quantity or type of CNT(s), having the same or different physicalproperties, or having the same or different functional properties, fromthe CNTs of the first aqueous solution. The CNT, suspended fibers ormaterials of the second aqueous solution can include other nano-sized ormicron-sized fibers or materials. The resulting CNT sheet 350 is driedand taken up on product roller as earlier described. FIG. 9 also showsthat a secondary veil layer 192 can be provided before the first headbox140. Similarly, and optionally, a second veil layer 292 can be providedbefore the second headbox 240 to provide a laminate of the first 192 andsecond veil 192 layers with the CNTs structures from the first headbox140 therebetween, and the CNTs structures from the second headbox 240thereon; and a third veil layer 392 can be provided before the thirdheadbox 240 to provide a laminate of the first 192, second 292 and third392 veil layers with the CNTs structures from the first headbox 140 andthe second headbox 240 therebetween, and the CNTs structures from thethird headbox 340 thereon. Optionally a fourth veil layer (not shown)can be applied on the resulting CNT sheet 350.

In an example embodiment, a first headbox 140 deposits a dispersed CNTsolution comprising entanglable CNTs, to form a base layer ofentanglable CNTs on the scrim. A second headbox 240, downstream of thefirst headbox 140, deposits a second dispersed CNT solution comprisingshort MWCNTs, which are distributed and filtered (211) as a second layerof short MWCNTs over the base layer of entanglable CNTs. A third headbox340 deposits a third dispersed CNT solution comprising entanglable CNTs,which are distributed and filtered as a top layer of entanglable CNTsover the second or intermediate layer of short MWCNTs.

In a second embodiment, a first headbox 140 deposits a dispersed CNTsolution comprising entanglable CNTs, to form a base layer ofentanglable CNTs on the scrim. A second headbox, downstream of the firstheadbox, deposits a second dispersed CNT solution comprising shortMWCNTs, which are distributed and filtered (211) as a second layer ofshort MWCNTs over the base layer of entanglable CNTs. A third headbox340 deposits a third dispersed CNT solution comprising thermoplasticfibers, which are distributed and filtered (311) as a top layer ofthermoplastic fibers over the second or intermediate layer of shortMWCNTs.

In other embodiments, any combination of A) entanglable CNTs, B) shortMWCNTs, and C) other or thermoplastic fibers, can be prepared innonwoven layers, ordered in any one of a wide variety, such as: ABA,ABB, ABC, ABC, CBA, CBC, ACA, etc.

In another embodiment of the invention, the second headbox can comprisea treatment solution which can be applied over the filtered nonwovenstructure formed from the first headbox. A non-limiting example caninclude an acidic solution which is passed through the CNT non-wovenstructure, and maintained for a period of time sufficient to effect achange in the functional properties of the CNTs. An option third headboxcan comprises a rinsing solution to remove the residual treatment (acid)solution following the (acid) treatment.

The second headbox, and subsequent headboxes, can be positioneddownstream from the first vacuum box by a distance sufficient to ensureeffective web formation, and to prevent any deconstruction of the firstdeposited layer of CNTs, or other fiber or material, deposited by thefirst (or previous) headbox, within the second (or subsequent) headbox.

In an alternative embodiment, the two or more headboxes can be arrangedside-by-side, in parallel, over the mesh screen, with correspondingvacuum boxes, in order to deposit different nonwoven materials ontoseparate lateral zones of the filter material. Second and subsequentdownstream headboxes can also be provided.

In an example, a wet-laying nonwoven apparatus has three headboxes andthree vacuum boxes, for wet laying in series of three liquid suspensionsof fibers. As a non-limiting example, the first and third suspensionsare an aqueous solution of acid-treated entanglable CNTs. The secondsuspension is an aqueous solution of short MWCNTs. The resultingnonwoven CNT includes a layer of short MWCNTs sandwiched between twolayers of entanglable CNTs.

We claim:
 1. A continuous process for manufacturing a continuouscomposite CNT sheet, comprising the steps of: i) applying a continuousporous substrate layer to an upper side of a continuous, moving porouscarrier material, and passing the continuous, moving porous substratelayer and the continuous, moving porous carrier material along a lengthof a top surface of a table, the table comprising a pooling region and avacuum zone downstream of the pooling region; ii) providing an aqueousSWCNT suspension comprising entanglable single wall carbon nanotubes(SWCNTs) in a dispersing liquid, the SWCNTs having an aspect ratio of atleast 2,500:1; iii) applying the aqueous SWCNT suspension onto the uppersurface of the continuous, moving porous substrate layer within thepooling region of the top surface of the table, to form a continuouspool consisting of the SWCNT suspension within the pooling region overthe continuous, moving porous substrate layer, the continuous poolhaving a depth sufficient to prevent puddling of the SWCNT suspensionupon the surface of the continuous, moving porous substrate layer; iv)passing the continuous, moving porous substrate layer and thecontinuous, moving porous carrier material, and flowing the continuouspool of the SWCNT suspension, along the pooling region; v) passing thecontinuous, moving porous substrate layer and the continuous, movingporous carrier material, and flowing the continuous pool of the SWCNTsuspension from the pooling region and over the vacuum zone; vi) drawingby vacuum in the vacuum zone only substantially all of the dispersingliquid of the continuous pool through the continuous, moving poroussubstrate layer and the continuous, moving porous carrier material, andfiltering a uniform dispersion of SWCNTs over the upper surface of thecontinuous, moving porous substrate layer to form an entangled SWCNTstructure; vii) drying any residual dispersing liquid from the entangledSWCNT structure over the continuous, moving porous substrate layer toform a composite SWCNT sheet over the continuous, moving porous carrierlayer, the composite SWCNT sheet including the entangled SWCNT structureand the continuous porous substrate layer; and viii) removing thecomposite SWCNT sheet from the continuous porous carrier material. 2.The process according to claim 1, wherein the continuous porous carriermaterial comprises a woven or meshed synthetic hydrophobic orhydrophilic polymer that filters the dispersed SWCNTs from thedispersing liquid of the aqueous suspension of the SWCNTs.
 3. Theprocess according to claim 1, wherein the entangled SWCNT sheet has abasis weight of at least 1 gram SWCNT per square meter (gsm), and up toabout 40 gsm, and a relative density to water of less than about 1.5. 4.The process according to claim 1, wherein a filler or additive isincluded within the SWCNT suspension.
 5. The process according to claim4, wherein the filler is a conductive filler selected from the groupconsisting of graphene, carbon nanofiber, nickel coated carbon fiber,nickel nanostrands or wires, silver nanowire or nanoparticles, goldnanowires or particles, and a mixture thereof.
 6. The process accordingto claim 4, wherein the filler is a non-conductive filler selected fromthe group consisting of ceramic powders or fibers, thermoplastic fibersor powders, thermoset fibers or powders, and a mixture thereof.
 7. Theprocess according to claim 1, wherein the continuous substrate layer isconductive, and is selected from the group consisting of carbon fiber,metallic materials selected from the group consisting of copper wires,foils, expanded foils, meshes and wovens, and metallized wovenmaterials.
 8. The process according to claim 1, wherein the continuoussubstrate layer is comprised of an electrically non-conductive material.9. The process according to claim 1, wherein the continuous poroussubstrate layer has a width up to about 152 cm (60 inches).
 10. Theprocess according to claim 1, further comprising the step of applying alayer of metal compound to a surface of the resulting composite SWCNTsheet, thereby increasing conductivity thereof.
 11. The processaccording to claim 10 wherein the step of applying a layer of metalcompound is a batch treatment process or a continuous process, selectedfrom the group consisting of sputtering, physical vapor deposition,pulsed laser deposition, electron beam, chemical vapor deposition,electro-chemical, electroplating, and electroless coating.
 12. Theprocess according to claim 1, wherein in the step iv), the continuouspool of the SWCNT suspension flows along the pooling region at avelocity of two or more times the linear velocity of the continuous,moving porous carrier material.
 13. The process according to claim 3,wherein the relative density of the entangled SWCNT sheet is less thanabout 1.0.
 14. A continuous process for manufacturing a continuouscomposite CNT sheet, comprising the steps of: i) applying a continuousporous substrate layer to an upper side of a continuous, moving porouscarrier material, and passing the continuous, moving porous substratelayer and the continuous, moving porous carrier material along a lengthof a top surface of a table, the table comprising a pooling region and avacuum zone downstream of the pooling region; ii) providing an aqueousSWCNT suspension comprising entanglable single wall carbon nanotubes(SWCNTs) in a dispersing liquid, the SWCNTs having an aspect ratio of atleast 2,500:1; iii) applying the aqueous SWCNT suspension onto the uppersurface of the continuous, moving porous substrate layer within thepooling region, to form a continuous pool consisting of the SWCNTsuspension within the pooling region over the continuous, moving poroussubstrate layer, the continuous pool having a depth sufficient toprevent puddling of the SWCNT suspension upon the surface of thecontinuous, moving porous substrate layer; iv) passing the continuous,moving porous substrate layer and the continuous, moving porous carriermaterial, and flowing the continuous pool of the SWCNT suspension, alongthe pooling region; v) passing the continuous, moving porous substratelayer and the continuous, moving porous carrier material, and flowingthe continuous pool of the SWCNT suspension, from the pooling region andover the vacuum zone; vi) drawing by vacuum in only the vacuum zone,substantially all of the dispersing liquid of the continuous poolthrough the continuous, moving porous substrate layer and thecontinuous, moving porous carrier material, and filtering a uniformdispersion of SWCNTs over the upper surface of the continuous, movingporous substrate layer to form an entangled SWCNT structure; vii)optionally drying any residual dispersing liquid from the entangledSWCNT structure over the continuous, moving porous substrate layer toform a composite SWCNT sheet over the continuous, moving porous carrierlayer, the composite SWCNT sheet including the entangled SWCNT structureand the porous substrate layer; and viii) removing the composite SWCNTsheet from the continuous, porous carrier material.
 15. The processaccording to claim 14, wherein the continuous porous carrier materialcomprises a woven or meshed synthetic hydrophobic or hydrophilic polymerthat filters the dispersed SWCNTs from the dispersing liquid of theaqueous suspension of the SWCNTs.
 16. The process according to claim 14,wherein the entangled SWCNT sheet has a basis weight of at least 1 gramSWCNT per square meter (gsm), and up to about 40 gsm, and a relativedensity to water of less than about 1.5.
 17. The process according toclaim 16 wherein the relative density of the entangled SWCNT sheet isless than about 1.0.
 18. The process according to claim 14, wherein thecontinuous substrate layer is conductive, and is selected from the groupconsisting of carbon fiber, metallic materials selected from the groupconsisting of copper wires, foils, expanded foils, meshes and wovens,and metallized woven materials.
 19. The process according to claim 14,wherein the continuous substrate layer is comprised of an electricallynon-conductive material.
 20. The process according to claim 14, whereinthe continuous porous substrate layer has a width up to about 152 cm (60inches).
 21. The process according to claim 14, wherein in the step iv),the continuous pool of the SWCNT suspension flows along the poolingregion at a velocity of two or more times the linear velocity of thecontinuous, moving porous carrier material.